In the image system field (hereinafter, called an image system) including a video surveillance system and an image delivery system, the need for an image transmission unit transmitting image data on an IP (Internet Protocol) network such as the Internet or LAN (Local Area Network) as a transmission line is increasing rapidly. In such image system, a combination of a camera and an image transmission unit or a network camera (referring to a camera integrated with an image transmission unit) is used and a recorder (hereinafter, called a video server) directly recording image data therefrom via a network is becoming mainstream. The above-described image transmission unit is a unit which, in order to transmit an image picked up by a camera to a network, compresses the image by MPEG2 (Moving Picture Expert Group Phase 2) or MPEG4 to convert it to image stream data (hereinafter, called image data).
A prior art camera transmits image data in an analog form of the NTSC system as a TV standard system using a coaxial cable. The camera transmits the image data as an analog signal so that deterioration due to attenuation or noise occurs during transmission to degrade image quality. In addition, a coaxial cable must be laid for each camera. The cable laying cost is tremendous with longer distance. A remote surveillance system in the actual use is significantly limited.
For image data from the above-described network camera or image transmission unit, an IP network such as the Internet or LAN has already been widely laid in a building or an area and no laying work laying a new IP network is necessary so as to build an image system very easily. As a result, connection to the Internet allows local image data to be viewed very easily in Japan and overseas. When laying a new IP network, wireless LAN is used to just place a relay station. No work providing a transmission line to a camera is necessary to significantly reduce the laying cost.
General personal computer peripheral equipment can be used as-is as equipment for relay transmission. It is much inexpensive than an analog camera. Because of an IP network system, system design in conjunction with various kinds of network components (such as a sensor, personal computer and server) is simplified. For example, in an entering/leaving management system in a building using an IC card, face photograph image data of an entering/leaving person is picked up by a network camera, and then, the frame image data of the person in the face photograph is stored together with an entering/leaving management database to build a high level of security network system in conjunction with the entering/leaving information and the image data.
The image system using an IP network has many advantages as described above. Since the technique and parts as a base are personal computer peripheral equipment, the image system has reliability lower than that of the analog camera system. Unlike the NTSC system, the IP network as a base has an asynchronous communication system assuming data loss, for example, packet discard when transmitting image data in a packet. A computer virus or sudden network load occurs to temporally interrupt image data. At worst, the entire image system can be down.
The personal computer peripheral equipment has equipment level more unstable than that of the analog camera system. Along with higher function and complicating of video equipment such as a camera or a recorder, a high-function and complicated OS (Operating System) such as Windows (Microsoft's trademark) or Linux is employed, and freezing of the image system is found here and there. When requiring a high degree of reliability in the image system, the network or equipment must be subject to duplicating or clustering like the server of a data center.
In the “duplicating” herein, another server performing quite the same function as a generally mainly operated server is prepared for backup, and the main server, which has failed, is switched to the backup server for operating the system continuously, after failure. The backup server must always be subject to mirroring of the main server. The mirroring herein means that the backup server into which the same data as that of the main server is written performs the same operation as that of the main server.
The “clustering” means that in a server configuration system for balancing load concentrated on one server, a plurality of node servers (servers performing actual processing) and a load balancer are used to configure one virtual server. That is, a plurality of node servers consist of Cluster. In the clustering, a failed server is disconnected at failure, and then, the entire another plurality of servers shares the processing of the failed server to operate the system continuously after failure. To apply the clustering, individual node servers must have an equal function and must be subject to mirroring mutually. In addition, each job is independent (or each session is independent) like Web servers to limit the use of service in which no state management is necessary between a client and a server.
In Web servers in which the duplicating technique and the clustering technique are standardly used, each job is basically independent in operation and no state management need be performed between a specific client and server. In the event that fail over occurs, the client can smoothly continue browsing without being affected thereby. The fail over herein means that two servers are operated at the same time and one of them is operated as a main server so that when the main server cannot be operated, the second server quickly takes over the processing as backup.
As a conventionally known multimedia recorder, for example, Japanese Patent Laid-Open Application No. Hei 11-250578 describes a plurality of recording devices and a backup device. The technique shows a technique enabling backup creation without deteriorating fast responsiveness at recording required for event recording.